286 



MICROBIOLOGY OF WATER AND SEWAGE 



what reduces the efficiency. From all this it is apparent that a filter 

 is a biological culture medium upon which the various types of bacteria 

 are growing and carrying out their functions and that such a medium 

 requires careful control and is sensitive to unfavorable changes in 

 environment (Fig. 117). 



The other filters are similar to this and illustrate the true function of 

 filtration. In the case of the sand filter it might be maintained that 

 filtration or straining was an essential element in the process, but in the 

 case of these coarse-grained media straining action is eliminated. Here 

 there is nothing but a pile of stones, varying from i t03 in ches or 

 more in .diameter, upon the surface of which the bacteria grow. The 



Fig. 118. — Sketch of septic tank. (Original.) 



sewage trickles slowly over the surfaces, or is held in contact with them 

 temporarily, according as we are dealing with trickling or contact filter?. 

 Solids adhere to the stones or settle upon them, and soluble material is 

 "absorbed" by the surface growth and removed from solution. Within 

 these gelatinous growths to which the air also has free access, the proc- 

 esses of oxidation take place and the products, the semi-oxidized 

 organic material, are later "shed" from the stones appearing again in 

 ~ the effluent as humus or stable organic matter. 



Anaerobic Tanks. — The cultivation of bacteria in anaerobic tanks 

 is not quite as simple a matter as that which has just been described. 



